Dental X-rays given to millions every year may dramatically increase the risk of thyroid cancer, scientists warned last night.

Researchers found that patients who had been X-rayed by their dentist at least ten times were more likely to develop the disease.

They have now warned that X-rays should not be given at check-ups or when registering new patients - despite these practices being common in many dental surgeries.

With rates of thyroid cancer more than doubling in 30 years, the scientists said that the potential dangers of dental X-rays were often overlooked.

Researcher Dr Anjum Memon, of Brighton and Sussex Medical School, said: 'Our study highlights the concern that, like chest or other upper body Xrays, dental X-rays should be prescribed when the patient has a specific clinical need, and not as part of routine check-up or when registering with a dentist.'

Dental leaders recommended that patients protect their thyroid - a hormone-releasing gland at the base of the neck - by wearing lightweight lead collars or bibs when being X-rayed.

The researchers asked 313 thyroid cancer patients and a similar number of healthy volunteers how many dental Xrays they had undergone.

After factoring in any hospital X-rays participants had had, they found that men and women who had had up to four dental X-rays were more than twice as likely to have developed the disease than those who had never had any.

Between five and nine X-rays and their risk rose more than four-fold, the journal Acta Oncologica reports.

In most danger were those who had had ten or more X-rays - their risk was 5.4 times that of someone who had never been X-rayed in the dentist's chair.

The researchers relied on patients' recollections, rather than dental records, but said that despite this, the finding was significant.

Dr Memon added that the results were supported by previous reports of increased risk of thyroid cancer in dentists, dental assistants and X-ray workers, suggesting that multiple low-dose exposures may be harmful.

Dr Memon, who carried out the research with experts from Cambridge and Kuwait universities, said: 'It is important that our study is repeated with information from dental records including frequency of X-rays, age and dose at exposure.

'If the results are confirmed, then the use of X-rays as a necessary part of evaluation for new patients, and routine periodic dental radiography, at six to 12 months interval, particularly for children and adolescents, will need to be reconsidered, as will a greater use of lead collar protection.'

But British dentists pointed out that the study was carried out in Kuwait, where rates of thyroid cancer are much higher than in the UK, and said the researchers did not know what sort of X-ray equipment had been used.

Dr Nigel Carter, chief executive of the British Dental Health Foundation, said: 'The number of X-rays being taken in dental practice in the UK has greatly reduced in recent years and the dosages from modern equipment are extremely low.

'Current recommendations would also be that a thyroid shield is always used to protect the patient against any spread of radiation. Dental X-rays have a valuable role in the diagnosis of dental disease.'